Trouble is almost certain if the far-Right leader Marine Le Pen and her far-Left counterpart Jean-Luc-Mélenchon are the two candidates who make it through to the second and final round on May 7, according to a report issued by French intelligence services.

The confidential document, leaked to Le Parisien newspaper, said that at the top of the list of potential security problems as millions of France cast their ballots was the “jihadist threat.”

The report came just two days after a French jihadist claiming allegiance to the Islamic State (Isil) shot dead a policemen on the Champs Elysées avenue in Paris, bringing election campaigning to an early end and thrusting security issues back to the top of the political agenda.

Around 50,000 police officers and 7,000 soldiers will be deployed to protect voters around France on Sunday for the first round, which has turned into a four-way race between Ms Le Pen, Mr Mélenchon, the scandal-scarred conservative François Fillon, and the maverick centrist Emmanuel Macron.

An opinion poll conducted on Thursday and Friday showed Ms Le Pen and Mr Macron tied on 23 percent, ahead of Mr Mélenchon with 19.5 per cent and Mr Fillon on 19 per cent.

But due to the margin of error pollsters factor in, there is no safe bet as to which two will make it to the second round of what has so far been the most unpredictable French presidential election in decades that comes in the wake of the UK’s shock decision to leave the EU and Donald Trump's presidential triumph in the US.

The French intelligence report leaked to Le Parisien said that spontaneous demonstrations - which might turn violent - could be held in major cities and troubled banlieues after the results are announced at around 7pm UK time on Sunday.

The report spoke of “public disturbances in the case of the presence (in the second round) of parties which are said to be extremist,” a reference to Ms Le Pen and the Communist-backed firebrand Mr Mélenchon. “In this case, protests are almost certainly to be expected,” it said.

The document also warned of farmers, hopsital staff and students taking to the streets to protest against the results. One in four voters is still undecided, according to polls that also said the French are more worried about jobs and the economy than terrorism.

The anti-EU and anti-immigrant Ms Le Pen moved quickly to present herself as the strongest defender against Islamist radicals in a country under a state of emergency since a string of terror attacks that began in 2015 and have killed more than 230 people.

"This war against us is ceaseless and merciless," she said, accusing President François Hollande’s Socialist government of a "cowardly" response to the threat.

Mr Macron and Mr Fillon also hastily convened televised briefings in which they vowed to protect the country.

Mr Mélenchon was the only one of the four to stick to his schedule after Thursday’s attack, which came just days after two men were arrested in Marseille on suspicion of planning an imminent attack on one of the presidential candidates.

He called for a "Europe of rebels", during a rally on Friday evening in Paris with Pablo Iglesias, the head of Spain's far-left Podemos party.

"Several Europes are possible, it doesn't have to be just their Europe," said Mr Mélenchon, a eurosceptic who has pledged to renegotiate treaties with the bloc.